This invention relates generally to devices for collecting and storing solar radiation, and particularly to a solar collector of the type in which solar radiation is focussed onto a light absorbing surface.
Prior art solar collectors of the focussing type have been known for many years. Examples of such collectors, utilizing lenses, are given in U.S. Pat. Nos. 3,929,121, 3,981,295, and 4,137,899. Each of these patents discloses a system in which a flat lens or a Fresnel lens focusses solar radiation upon a conduit containing a working fluid. Typical systems of this type are unable to maintain continuous focussing of the sun's rays upon the conduit without complicated and expensive tracking equipment. See, for example, the gear and hinge arrangement illustrated in U.S. Pat. No. 3,929,121.
An attempt to solve this problem is discussed in U.S. Pat. No. 4,033,324 which uses a number of spherical lenses focussing the sun's radiation onto a substrate below the lenses. This structure is less than satisfactory, however, since the sun's rays will not be properly focussed upon the underlying substrate, except during a short portion of the day when the angle of incidence of the sun's rays properly corresponds to the particular distance of the substrate from the center of the spherical lens.
Another patent disclosing a panel of spherical lenses is U.S. Pat. No. 2,277,311. In this reference, hemispherical lenses focus the sun's rays onto a set of opposing hemispherical lenses which diffuse the rays onto a substrate. Although this device will collect radiation from the sun over a large portion of the day, there does not appear to be any effective focussing of the rays onto a collector.
A more interesting approach is taken in U.S. Pat. No. 1,093,498, in which focussing is accomplished using a number of spherical lenses, each positioned above a hemispherical collecting cup. Since the collecting cup is concentric with the spherical lens, the sun's rays will be focussed on some portion of the interior of the cup during the entire day. However, the collecting cups are fabricated from metallic radiating cases which both absorb and re-radiate heat through the lens; the device is therefore inefficient as a solar collector.
In U.S. Pat. No. 3,587,559, a similar configuration is disclosed in which the interior of the collecting cup is lined with a carbon impregnated cloth to increase the absorption of solar radiation thereon. Again, however, the net efficiency on the device is not increased since the collector will re-radiate from all portions of the darkened cup back through the spherical lens.